Power distribution switches are used in power management integrated circuits to distribute power to subsystems. In one example, power distribution switches are used to limit the current flowing to an output device. For example, in a computer system a power distribution switch may couple a power source to an output terminal, thereby supplying power to an attached device. Some devices are limited to 0.5 amperes (A) of current for extended periods, but the devices may occasionally draw more current than this limit. If a device overdraws current for too long, the power distribution switch may increase its resistance, thereby reducing the current draw and preventing the device from significantly affecting the voltage level of the power source.
Conventional power distribution switches include analog feedback loops to monitor the input to the power distribution switch for excessive current draw by a device drawing power from the output. In an example, a sense resistor is placed in series with the current supply to generate a sense voltage that is compared to a desired reference voltage at an amplifier. In such arrangements, the output of the amplifier controls the power distribution switch coupling the power source to the output terminal, and increases the resistance of the switch if the sense voltage indicates that too much current is being drawn. However, in the case of a hard short circuit condition (e.g., a condition in which the resistance between an output terminal of the power distribution switch and ground drops to roughly 0.1 ohms), large currents may flow through the power distribution switch before the feedback loop can respond to increase resistance of the power distribution switch. This condition may be damaging to the power distribution switch, or may lead to a situation in which the power source voltage is pulled below tolerable levels.
One conventional approach, described in U.S. Pat. No. 7,817,393 (incorporated by reference herein in its entirety), addresses the undesired slow response to hard short by adding a digital voltage feedback loop that responds to the hard short more rapidly than the analog feedback loop. In response to detection of a hard short, the digital loop disables, and/or increases the resistance of, one or more conduction paths in the power distribution switch, thereby increasing its series resistance. For example, if the current limit of the switch is set to 1 A, the digital loop may rapidly disable ⅘ of the conduction paths, thereby limiting the current to 200 mA. This condition persists until the output terminal voltage recovers from the hard short. However, in some situations, the load device attached to the output terminal may require full current (e.g., significantly more than 200 mA) for start up, such that the output terminal voltage cannot recover while the 200 mA current limit condition persists.
It is desirable in view of the foregoing to provide for rapid current limit protection against shorts without requiring the output terminal voltage to recover before the current limit condition can be removed.